linux-kernel - Re: [PATCH 2/3] hwrng: stm32 - add support for STM32 HW RNG

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Date:	Sun, 11 Oct 2015 20:24:08 +0100
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Lee Jones <lee.jones@...aro.org>, Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Patch Tracking <patches@...aro.org>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>
Subject: Re: [PATCH 2/3] hwrng: stm32 - add support for STM32 HW RNG

On 5 October 2015 at 10:22, Daniel Thompson <daniel.thompson@...aro.org> wrote:
> On 4 October 2015 at 11:32, Linus Walleij <linus.walleij@...aro.org> wrote:
>> On Sat, Oct 3, 2015 at 10:35 PM, Daniel Thompson
>> <daniel.thompson@...aro.org> wrote:
>> Then this construct:
>>
>>> +static int stm32_rng_read(struct hwrng *rng, void *data, size_t max, bool wait)
>>> +{
>> (...)
>>> + /* enable random number generation */
>>> + clk_enable(priv->clk);
>>> + cr = readl(priv->base + RNG_CR);
>>> + writel(cr | RNG_CR_RNGEN, priv->base + RNG_CR);
>> (...)
>>> + /* disable the generator */
>>> + writel(cr, priv->base + RNG_CR);
>>> + clk_disable(priv->clk);
>>
>> This is in the hotpath where megabytes of data may be read out
>> by consecutive reads. I think it's wise to move the clock enable/disable
>> and also the write to cr to enable/disable the block to runtime PM
>> hooks, so that you can e.g. set some hysteresis around the disablement
>> with pm_runtime_set_autosuspend_delay(&dev->dev, 50);
>> pm_runtime_use_autosuspend(&dev->dev);pm_runtime_put_autosuspend().
>> For a primecell check the usage in drivers/mmc/host/mmci.c
>> for a complex usecase or e.g. drivers/hwtracing/coresight/coresight-tpiu.c
>> for a simpler usecase.
>>
>> For the performance hints I guess you can even test whether
>> what I'm saying makes sense or not by reading some megabytes
>> of random and timing it.
>
> I'll have to benchmark this. clk_enable/disable have pretty simple
> implementations on STM32 so there might not be much in it.

Well... I was extremely wrong about that!

Switching the driver over to autosuspend yielded a very significant
performance boost: ~1.1MiB/s to ~1.5MiB/s .

Very pleased with that!


Daniel.
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